Beijing might soon declare an air defense zone in the disputed South China Sea

Chinese dredging vessels are purportedly seen in the waters around Mischief Reef in the disputed Spratly Islands in the South China Sea in this still image from video taken by a P-8A Poseidon surveillance aircraft provided by the United States Navy May 21, 2015. REUTERS/U.S. Navy/Handout via Reuters China is preparing an air defence identification zone (ADIZ) in the South China Sea, two years after it announced a similar one in the East China Sea, according to sources close to the People's Liberation Army and a defence report.

But one source said the timing of any declaration would ­depend on security conditions in the region, particularly the United States' military presence and diplomatic ties with neighboring countries. "If the US military keeps making provocative moves to challenge China's sovereignty in the region, it will give Beijing a good opportunity to declare an ADIZ in the South China Sea," the source said.

The revelation came ahead of the Shangri-La Dialogue in Singapore, a security forum attended by defence officials from various nations, including Admiral Sun ­Jianguo and US Secretary of ­Defence Ash Carter. Disputes in the South China Sea are expected to head the agenda of the three-day event, which starts on Friday.

Top Chinese and US officials will also meet next week for their annual strategic and economic dialogue in Beijing.

In a written response to the South China Morning Post on the zone, the defence ministry said it was "the right of a sovereign state" to designate an ADIZ.

"Regarding when to declare such a zone, it will depend on whether China is facing security threats from the air, and what the level of the air safety threat is," the statement said.

Japan Maritime Self-Defense Force's PC3 surveillance plane flies around the disputed islands in the East China Sea, known as the Senkaku isles in Japan and Diaoyu in China, October 13, 2011. REUTERS/Kyodo

China set up its first ADIZ in the East China Sea in November 2013 to cover the Diaoyu Islands, which Japan calls the Senkakus. Both countries claim the uninhabited outcrops but Tokyo controls them. The ADIZ triggered a backlash from Japan, South Korea and the US.

Tensions between China and neighbors Vietnam, Malaysia, Brunei and the Philippines over sovereignty in the South China Sea have risen since Beijing ­embarked on major land reclamation work on disputed islands and reefs in the area.

A report in Canada-based Kanwa Defence Review said Beijing had defined the area of the ADIZ in the South China Sea, and the timing of the announcement would be a political decision.

The report said the new ADIZ would be based on the exclusive economic zone (EEZ) of Woody Island and China's seven new artificial islands in Spratly chain, or 200 nautical miles stretches from the islands' baseline.

"China's new ADIZ will overlap with the EEZs of Vietnam, the Philippines and Malaysia, which are also planning their own ADIZs - with US backing - if China ­announced it," Kanwa editor-in­chief Andrei Chang said.

Ni Lexiong, a Shanghai-based military commentator, said the seven artificial islands in the Spratly chain had laid the foundations for China to establish its ­ADIZ in the South China Sea.

But Beijing-based naval expert Li Jie said there were signs that ­regional tension would ease after Rodrigo Duterte became president of the Philippines.

President Xi Jinping sent a congratulatory message to Duterte on Monday, saying China hoped "the two sides can work together to bring bilateral relations back on a healthy track."